WCDMA (Wideband Code Division Multiple Access) provides communications capability in both a FDD (Frequency Division Duplex) mode and a TDD (Time Division Duplex) mode. In the TDD mode, system capacity is maximized when the base stations (Node Bs) in a UTRA (UMTS Terrestrial Radio Access) TDD mode deployment area operate synchronously. But this may not be the case, and there are other possible deployment scenarios in areas used for mobile communications. Multi-operator, single operator, unsynchronized network, synchronized network with asymmetry on the DL/UL (downlink/uplink) and other access technology are often used in the same area. In systems where different access technologies are used, base stations and mobile terminals generate considerable noise to other radio nodes and systems as illustrated in FIG. 1.
Due to non-zero time phenomena, even a synchronized network generates unwanted noise to other base stations within its own system due to DL propagation time over a distance as illustrated in FIG. 2. In FIG. 2, a communications network includes base stations BS 1 and BS 2. In this network, the BS 1's DL (signal transmission from the base station to the mobile terminal) communication interferes with a portion of the BS 2's UL (signal transmission from the mobile terminal to the base station) communication.
The noise exists in both the FDD and TDD modes. In the FDD mode, different frequency bands are used for UL and DL allowing both the UL and DL to operate continually. When the UL communications from the mobile terminals are continual, then the system knows approximately the total power of the signals, wanted and unwanted, received by the base station.
In the TDD mode, the same frequency band is used for both UL and DL communications. That is, the DL and UL communications are separated by time slots instead of frequency bands in the TDD mode. It is not unusual to have asymmetry in DL/UL traffic time in a TDD system. That is a majority of the time slots can be devoted to DL communication and a minority of time slots can be devoted to UL communication or vice versa.
Automatic gain control (or simply “gain control”) is an important function in mobile communication systems. The purpose of the automatic gain control is to reduce or attenuate a strong interferer, blocker, or other noise that comes to the system. One gain control function is to dynamically adjust the gain of the incoming signals during receiving so that saturation is prevented or minimized. The incoming signal into the base station's Rx includes a desired signal and the noise signal. The gain control should find a balance of signal level (power level) in the Rx chain so that an analog-to-digital converter (ADC) in the Rx chain works near its maximum input level, without going to saturation. This maximizes the dynamic range of the system.
In existing FDD systems, AGC process may take about 2-3 ms. As described in the document “3GPP TS 36.211 V1.2.0” from the 3rd Generation Partnership Project, both DL and UL communications are organized into radio frames each with a 10 ms duration. Two types of radio frame structures are supported. Type 1 is applicable to both FDD and TDD and type 2 is applicable to TDD only.
In type 1 TDD systems illustrated in FIG. 3, each radio frame is divided into to 20 time slots (numbered 0-19) with each time slot being 0.5 ms in duration. A subframe consists of two consecutive time slots such as time slots 0 and 1. Thus, each subframe is 1 ms long. Each subframe is allocated to be used for either the UL or DL communication with the exception of subframe 0 (time slots 0 and 1) and subframe 5 (time slots 10 and 11) which are reserved for DL.
In type 2 systems illustrated in FIG. 4, each radio frame is divided into two half-frames of 5 ms each. Each half-frame includes 7 times slots (numbered 0-6), each of which are 0.675 ms long in duration. Each half-frame also includes three special fields—DwPTS (Downlink Pilot Time Slot), GP (guard period) and UpPTS (Uplink Pilot Time Slot)—between time slots 0 and 1. In each time slot, the communication can be either UL or DL. However, time slot 0 and DwPTS are reserved for DL and UpPTS and time slot 1 are reserved for UL.
In the TDD systems, a particular amount of time (1.0 ms for type 1 and 0.675 ms for type 2) is allocated for UL communication. But as noted above, the reaction time of the gain control in existing FDD systems is on the order of 2-3 ms. This means that for the TDD systems, the existing gain control cannot react fast enough to control the gain during any one allocated subframe or time slot in which the UL communication takes place.